1. Field of the Invention
The present invention relates to acoustic shaping devices. More particularly, embodiments of the invention provide devices and methods for shaping the acoustic spectrum passing through a device.
2. Description of Related Art
As one example of an acoustic shaping device, conventional hearing protection devices (HPDs) are generally delineated into passive (non-electronic) and active (powered electronic) designs. See Casali, J. G., Advancements in hearing protection: Technology, applications, and challenges for performance testing and product labeling. Proceedings of the 2005 International Congress and Exhibition of Noise-Control Engineering, Rio de Janeiro, Brazil, 2097-2118 (2005) (“Casali 2005”). Passive HPDs are further categorized as earplugs, ear canal caps, earmuffs, and helmets, based on the location of the sealing and design configuration. See Gerges, S. and Casali, J. G., Ear protectors, in Crocker, M., Ed. Handbook of Noise and Vibration Control, John Wiley, New York, 31, 364-376, (2007) (“Gerges and Casali, 2007”).
Passive HPDs generally attenuate noise through static, passive means. Most hearing protection fails to deliver a flat attenuation (that is, equivalent or near-equivalent reduction of sound) across the frequency spectrum. Instead, attenuation typically increases in decibels (dB) as the frequency increases in a non-linear manner. This non-linear behavior affects the perception of sound frequencies across the audible spectrum in different degrees and creates an unnatural imbalance in perceiving sound pitch. As a result, pitch perception and other auditory experiences which rely on frequency-based cues can be compromised by the non-linear attenuation imparted by conventional hearing protectors.
For example, earmuffs, on average more so than earplugs, tend to attenuate high-frequency sound more than low frequency sound, thereby reducing the power of consonant sounds that are important for word discrimination and which lie in the higher frequency range (Gerges and Casali, 2007). The sloping, nonlinear attenuation profile versus frequency, which provides higher attenuation values with increasing frequency, creates a spectral imbalance from the listener's perspective. This imbalance occurs because the relative amplitudes of different frequencies are heard differently than they would be without the conventional HPD, that is, with the open ear, and thus broadband acoustic signals are heard as spectrally different from normal. In other words, the sounds are often reported as more bassy and resonant. Thus, the spectral quality of a sound is altered, and sound interpretation, which is important in certain aural tasks, may suffer as a result. Thus, there is a desire for uniform (or flat) attenuation HPDs, since these devices tend not to bias the hearing of sounds across the audible frequency range.
Although on average, earmuffs display a larger imbalance between their low and high-frequency attenuation than do earplugs, some earplugs demonstrate substantial spectral nonlinearities in their attenuation. This is shown in the two lowermost functions of FIG. 2. When listening to a sound while wearing such conventional earplugs (or an earmuff), all pitches that compose the sound are reduced in level, but due to the influence of the nonlinear attenuation, the amplitudes of various pitches are also changed relative to one another in a non-uniform manner across the spectrum, rendering the wearer's hearing of the sound as distorted when compared to its perception with the unoccluded ear. Accordingly, in many situations there is a need for uniform or “flat” attenuation, such as pitch perception by musicians, cutting speed/friction by machinists, impending bearing failures by helicopter pilots, and “roof talk” by underground miners to name a few.
In an attempt to counter these effects, in the early 1990s, flat or uniform attenuation HPDs were developed by AEARO-3M and Etymotic Research, Inc., including the Etymotic ER-15 Musician's custom-molded earplug and the ER-20 HiFi™ pre-molded earplug. See Casali, J. G. and Berger, E. H. Technology advancements in hearing protection: Active noise reduction, frequency/amplitude-sensitivity, and uniform attenuation. American Industrial Hygiene Association Journal, 57, 175-185. (1996) (“Casali 1996”). As illustrated in FIGS. 3A-B, these devices use acoustical damping and filtering networks, as well as unique placement of the sound entry port near the ear canal's rim, to provide essentially flat attenuation over the range of frequencies from 125 Hz to 8000 Hz. Attenuation of these devices is shown in FIG. 2 (uppermost functions).
Another, different, class of HPDs are passive, level-dependent HPDs which are designed so that their attenuation increases as the ambient noise level increases. Such devices rely upon acoustical networks, mechanical ball or flutter valves, or orifices in blocked sound ports which respond dynamically to intense air pressure changes to activate their unique attenuation responses. One of the earliest designs in this category was a dynamically-valved earplug named the Gunfender™, and the North Safety Co. followed with a device called the Sonic Ear-Valve™. See Mosko, J. D. and Fletcher, J. L., Evaluation of the Gunfender earplug: Temporary threshold shift and speech intelligibility. Journal of the Acoustical Society of America, 49, 1732-1733. (1971). In the early 1990s, an additional earmuff style device that relied on a sharp-edged, orifice-based, controlled leakage path in a duct was E_A_R Corporation's Ultra 9000™. See Allen, C. H. and E. H. Berger: Development of a unique passive hearing protector with level-dependent and flat attenuation characteristics. Noise Control Engineering Journal, 34(3), 97-105. (1990) (“Allen 1990”). Later, using similar technology comprising a calibrated leaky filter in an acoustical duct running through the stem of an earplug, the AEARO Combat Arms™ earplug was developed for military use, which was followed by a recent commercial version named the Arc™ earplug. See Babeu, L. A., Binseel, M. S. Mermagen, T. J. and Letowski, T. R. Sound localization with the Combat Arms™ earplug. Proceedings (on CD) of the 29th Annual National Hearing Conservation Association Conference, Seattle, Wash., Feb. 19-21. (2004) (“Babeu 2004”); and Casali, J. G., Ahroon, W. A., and Lancaster, J. A field investigation of hearing protection and hearing enhancement in one device: For soldiers whose ears and lives depend upon it. Noise and Health Journal, 11(42), 69-90. (2009) (“Casali 2009”). A custom-molded earplug was introduced by Variphone Benelux N.V., the Variphone Stopgun™, which uses a nonlinear filter to attenuate impulsive noises of above about 110 dB according to the manufacturer's website: http://www.variphone.com/en/hearing-protection/shooting-sport/stopgun.
Typically, passive level-dependent HPDs provide very low attenuation in low to moderate noise levels; however, as ambient noise levels increase to a certain level, their attenuation increases to a maximum and plateaus afterwards. (Casali 1996; Allen 1990). With contemporary orifice/acoustical filter-based, level dependent HPDs at low noise levels, their passive attenuation behaves as that of a leaky protector, offering minimal attenuation below about 1000 Hz because laminar flow is present in the duct and sound passes with low impedance through the orifice. This minimal attenuation is all that is available to protect the wearer's hearing at sound levels below about 110 dB. Since such devices are intended to be used primarily in intermittent impulsive noise, this should not be a problem as long as the off periods are relatively quiet (e.g., below an A-weighted noise level of approximately 85 dB). At elevated sound pressure levels (above about 110 dB to 120 dB, as might occur during a gunshot), the flow through the orifice changes from laminar to turbulent, effectively closing the orifice and thus sharply increasing the attenuation of the device. (Allen 1990).
Due to the fact that level-dependent earplugs of the Combat Arms™ and Arc™ types provide very little protection at sound levels below an A-weighted noise level of about 110 dB, they are clearly not suitable for continuous noise exposures and are intended for intermittent exposures which entail quiet periods interrupted by sudden explosions, gunshots, arc blasts, high pressure pneumatic discharges, or similar impulsive sounds. However, to provide protection in situations wherein both intermittent quiet/impulsive noise as well as periods of continuous noise can manifest, both the Combat Arms™ and Arc™ earplugs were designed with two “ends” that afford selectable protective states, in which one end is level-dependent, and the other is a conventional passive earplug that is suitable for continuous noise exposures. (Babeu 2004; Casali 2009). An example of the first generation Combat Arms™ earplug with the level-dependent and conventional passive ends is shown in FIG. 4A. Now in its third generation, and as shown in FIG. 4B, a more recent version of the Combat Arms™ earplug incorporates a single end that is manually converted between the two aforementioned conventional and level-dependent states by a manually-operated, rocker-activated valve. An additional advantage, most relevant to attempts at producing spectral shaping as a function of frequency by mechanical means, is that some orifice-based, level-dependent HPDs, such as the AEARO-3M Ultra 9000 Earmuff™, offer roughly flat attenuation, though not the case with the level-dependent end of the Combat Arms™ earplug. See Allen 1990.
One consequence of improper attenuation in an HPD is that a user may reject the hearing protection if it compromises his/her hearing to the extent that sounds no longer appear natural, signals cannot be detected or localized, and/or speech cannot be understood. In some cases, too much attenuation may be provided by an HPD for a particular noise situation, with the concomitant effect that the user's hearing is unnecessarily degraded. In lay terms, this is commonly referred to as “overprotection.” The safety professional often faces a dilemma in selecting HPDs for the workforce. They must provide adequate attenuation for the noise threat at hand, but they may not provide so much attenuation that the worker cannot hear important signals and/or speech communications—the dilemma of underprotection versus overprotection.
A major stimulus for the development of augmented HPDs has been the sometimes negative influence that conventional HPDs have on the hearing ability of users. See Casali, J. G. and Gerges, S., Protection and enhancement of hearing in noise, in Williges, R. C., Ed. Reviews of Human Factors and Ergonomics, Vol. 2. Human Factors and Ergonomics Society Santa Monica, Calif., 7, 195-240, (2006) (“Casali 2006”); and Suter, A. H., The effects of hearing protectors on speech communication and the perception of warning signals (AMCMS Code 611102.74A0011), Aberdeen Proving Ground, Md.: U.S. Army Human Engineering Laboratory, 1-32. (1989) (“Suter 1989”).
To help overcome the problem of overprotection in moderate noise environments, earplug augmentations have been developed to allow the user some level of control over the amount of attenuation achieved. These devices incorporate a leakage path that is adjustable by setting a valve that obstructs a tunnel or “vent” cut through the body of the plug, or by selecting from a choice of available filters or dampers that are inserted into the vent.
The Variphone™ is one such example of an adjustable-valve design and is constructed from an acrylic custom-molded impression of the user's ear canal. The attenuation adjustment range of the device is approximately 20 dB to 25 dB below 500 Hz, with a maximum attenuation of about 30 dB at 500 Hz. At higher frequencies, the range of adjustment decreases, while the maximum attenuation attainable increases slightly.
The Sonomax SonoCustom™ is an example of a selectable-damper design. This device can be fitted with a variety of attenuation dampers that provide the opportunity for discretely variable attenuation in a single device, and each damper has distinct spectral attenuation values and NRR. Furthermore, the SonoCustom™ HPD is sold as a system with a probe tube microphone test apparatus which verifies the amount of attenuation achieved by way of MIRE techniques on each user as they are fit with the product.
There is also a full custom-molded option of the acrylic Variphone™ brand earplug as well as the silicone V-SIL™, both of which incorporate a duct into which selectable “filters” are inserted for different attenuation values. Another device is the dB Blocker™ from Custom Protect Ear. This product is a vented, custom-molded earplug that offers different cartridge filters that can be inserted into the vent. Each cartridge comprises a unique damper/filter which affords a specific attenuation spectrum, and the selection of cartridge is based upon an analysis of the wearer's noise exposure and other needs. The cartridge is intentionally not user-replaceable, so the dB Blocker™ is returned to the manufacturer should a cartridge need replacement or changing.
Two important distinctions between passive adjustable-attenuation HPDs and passive level-dependent HPDs is that the former require user or manufacturer setting to effect attenuation changes, and the attenuation, once selected, is essentially independent of incident sound level, that is, level-independent. On the other hand, level-dependent devices react automatically to changes in incident sound pressure levels and the user has no control over the change in attenuation when the HPD is worn in its level-dependent configuration.
Attenuation testing of adjustable attenuation passive devices is only slightly more complex than for flat-attenuation passive devices. For devices with discrete settings (e.g., the SonoCustom™ and the dB Blocker™), the EPA proposed rule specifies using the standard REAT test of ANSI S12.6-2008 for each level of adjustment (or for each damper/filter insert) and an NRR value is determined for each setting. Although this is time consuming and labor intensive, it is necessary protocol to quantify the performance at each setting or for each cartridge insert. Continuously variable devices (e.g., Variphone™) are more problematic to attenuation testing because they can only be tested reliably at the extremes of their adjustment range (i.e., fully open and fully closed). It is more difficult to reliably quantify the protection afforded by such devices at all intermediate settings, unless those intermediate settings are reproducible through a detent or graduation setting on the valve control.
The adjustable-attenuation class of HPDs affords flexibility in product development in that these devices can be designed to allow for modular augmentations, and this is potentially a major advantage in that these relatively expensive and personalized (i.e., custom- or semi-custom-molded) earplugs can then be adapted to changing user needs and different noise environments without making a new custom-molded earplug. Filter-based devices can be tuned for specific environments or tuned to pass speech or other critical bands necessary for specific jobs, assuming that the filter's passband response is properly optimized to the objective.
Due to the simple fact that a uniform HPD's relatively flat attenuation spectrum enables the listener's ears to retain their normal, albeit uniformly attenuated, frequency response, perceptual advantages of these specialized hearing protectors are obvious, and for certain user populations, such as musicians, the more natural hearing provided should prove to be beneficial. However, the purported benefits to hearing perception of flat attenuation HPDs have been tested in few studies. One notable exception was a demonstration experiment by Witt, who, in an effort to determine whether the presence of flat attenuation was noticeable by HPD users in industrial applications, recorded speech and industrial noise under varying attenuation slopes of earplugs and played them back to obtain subjects' responses. The benefits of near-flat attenuation (as achieved with a prototype of the Sperian AirSoft™ earplug) were most noticeable in industrial settings when the increase in earplug attenuation was less than a slope of 10 dB over the frequency range of 250 Hz to 4000 Hz. Furthermore, Witt noted that while the first flat attenuation devices developed in the 1990s (i.e., the ER earplugs discussed above) utilized controlled, tuned leakage paths and dynamic mechanical networks to yield their linear attenuation, advances in earplug materials in the first half of the 2000s decade have enabled near-linear attenuation in certain disposable earplugs, thus bringing the cost of uniform (or at least “near-uniform”) attenuation technology down into the realm of more industrial users. Witt, B. Can you hear flat?? Proceedings (on CD) of the 31st Annual National Hearing Conservation Association Conference, Tampa, Fla., Feb. 16-18. (2006).
To achieve near-uniform attenuation with disposable devices, however, the quality of fit is important, since an acoustical leak will invariably degrade (that is, increase) the low-frequency attenuation. It is also important to recognize that “true” flat attenuation HPDs (e.g., ER-15, ER-20) that incorporate the leakage paths and mechanical networks noted above provide generally lower attenuation than that afforded by most well-designed conventional earplugs, so they are not typically appropriate for ear defense in high exposure levels. (Casali 1996; Casali 2006).
Many user-molded, conventional passive earplugs have been successful in the hearing protection marketplace. Such products are typically designed to provide a “one-size-fits-most” earplug that is constructed from a malleable or compressible/expandable-recovery material and that is larger in cross-sectional diameter than the ear canal. Typically, these user-molded products are manufactured with materials such as slow-recovery polyurethane or polyvinyl foams, finely spun fiberglass (also known as Swedish Wool™), various paraffin and beeswax-based products, or malleable putty encapsulated inside a soft plastic sheath, which are formable and/or exhibit compressible/expandable-recovery. Since their original dimension is bigger than the ear canal into which they are inserted, the recovery process creates an acoustic seal. In order to achieve a quality fit, the user must first manually “mold” or form (by way of finger-exerted compression and/or elongation force) the earplug into an “undersize” shape before it is actually inserted, and then to quickly insert it before it returns to its original shape and size. For some users, this manipulation of the earplug prior to insertion and subsequent prompt insertion can be difficult. Furthermore, foam, putty, or wax-based earplugs cannot be “dynamically” adjusted inside the canal once they have been inserted. Instead, they must be fully removed from the ear canal, and then a new molding/insertion process must commence.
Foam earplugs, such as SparkPlug® by Moldex and E-A-R Classic PVC foam earplugs, are examples of slow-recovery devices. Premolded flanged earplugs such as Ultrafit® by AEARO-3M and HOWARD LEIGHT Fusion® only require users to push them into ear canal without any premolding. Earplugs made of paraffin or beeswax will require initial premolding by the user and then a forced insertion to deform them to fit the ear canal of the user.
Custom-molded earplugs can be made as either passive or active devices. See Casali, J. G., Passive Augmentations in Hearing Protection Technology Circa 2010: Flat-Attenuation, Passive Level-Dependent, Passive Wave Resonance, Passive Adjustable Attenuation, and Adjustable-Fit Devices: Review of Design, Testing, and Research. International Journal of Acoustics and Vibrations, 15(4), 187-195 (December 2010) (“Casali 12010”); Casali, J. G., Powered Electronic Augmentations in Hearing Protection Technology Circa 2010 including Active Noise Reduction, Electronically-Modulated Sound Transmission, and Tactical Communications Devices: Review of Design, Testing, and Research. International Journal of Acoustics and Vibrations, 15(4), 168-186 (December 2010) (“Casali II 2010”).
After a custom-molded earplug is produced for a person's ear canal, it can be used by itself as a passive device that provides relatively good attenuation if the impression is usually made with a deeply-inserted ear dam. Many companies, however, such as Sonomax and Custom Protect Ear, create a pass-through channel or vent that can be fitted with various dampers that can provide different levels of attenuation. The channel or vent can also be fitted with blocks that contain electronic circuitry that can provide electronic augmentations such as noise cancellation, electronic filtering, closed-loop attenuation control, hearing assistive circuits, automatic gain control, digital signal recognition/processing, and so forth. See Casali II 2010.
Thus, a one-size-fits-all type of device capable of producing a flat (that is, relatively uniform) attenuation across the acoustic frequency spectrum, as well as a device that can be easily modified through adjustments in certain design parameters to produce attenuation that is spectrally-shaped across the frequency spectrum for shapes other than flat, and/or a device which may afford user in-situ adjustability as to spectral attenuation, would be highly desirable.